1. Field of the Invention
The present invention relates to an optical equalizer and an optical amplifier and a wavelength multiple optical transmission apparatus using the optical equalizer, and more particularly to an optical equalizer for flattening gain wavelength dependence characteristics of an optical amplifier and an optical amplifier to which the optical equalizer is applied.
2. Description of Related Art
There is an optical wavelength multiple transmission system as one of the techniques for realizing a large capacity of an optical communication system. This is a system that transmission is made after a plurality of optical signals having mutually different wavelengths are multiplexed on one line of optical fiber.
On the other hand, an optical fiber amplifier using a rare earth element doped optical fiber as an amplification medium which has been realized rapidly in recent years can amplify the signals having different wavelengths collectively. When this optical fiber amplifier is applied to an optical wavelength multiple transmission system, a large capacity long distance transmission system can be realized.
Now, the gain is generally different depending on the wavelength of an optical signal with respect to either an optical fiber amplifier and an optical semiconductor amplifier. Accordingly, when a plurality of wavelength multiplexed signal lights are amplified collectively, the optical output level of the optical amplifier is different depending on the wavelength. Moreover, when optical amplifiers are connected on multistages, the level difference of the optical output is accumulated.
When reception is made after wavelength branching on a receiver side, problems of stroke deterioration among respective wavelengths and in point of setting a light receiving level of a receiver are produced since the optical output level is different depending on the wavelength. Thus, an optical equalizer for compensating gain wavelength dependency of an optical amplifier becomes necessary.
There has been heretofore a method of inserting an equalizing optical filter for flattening the gain wavelength dependency into an optical fiber amplifier as one of the methods for compensating the gain wavelength dependency of an optical fiber amplifier. Such a technique is described in Japanese Patent Application No. Hei 01-177985 for instance. Further, as another method, there is a method of providing an optical fiber coupler such as described in Japanese Patent Application No. Hei 05-109975.
Further, as one of equalizing optical filters, a grating type (such as M. Tachibana, R. I. Laming, P. R. Morkel and D. N. Payne "Gain-shaped Erbium-doped fibre amplifier with broad spectral bandwidth", Technical Digest on Optical Amplifiers and their Applications, pp. 44-47, 1990) and a Mach-Zehnder type (such as disclosed in Japanese Patent Application No. Hei 2-270766) are proposed.
Furthermore, as an improved type of a Mach-Zehnder type optical equalizer, that in which Mach-Zehnder type optical filters having mutually different variation periods of loss depending on wavelength of wave-length dependency of transmittance are cascade-connected on multistages has been proposed (such as disclosed in Japanese Patent Application No. Hei 5-60047).
The above-mentioned conventional examples have drawbacks shown hereunder, respectively.
Namely, an optical fiber coupler is produced by fusing and stretching two lines of optical fibers, in which evanescent coupling among optical fibers is utilized. In this sort of coupler, when a variation period of wavelength dependency of the loss is shortened, it is required in point of principle to produce a fusion coupling portion long, and the manufacturing reproducibility of loss wavelength characteristics is lowered with the length of the fusion coupling portion.
In a band of wavelength 1,550 nm used in an optical fiber amplifier at present, when the gain to wavelength characteristics of an optical fiber amplifier is going to be compensated, it is required to shorten the period of a loss to wavelength characteristic curve of an optical fiber coupler to approximately 100 nm or lower, and it is attended with technical difficulties to manufacture such characteristics stably.
Generally, when the gain to wavelength characteristics of an optical amplifier is going to be compensated with an optical filter, the transmission wavelength characteristics of an optical filter have to be made double-humped in many cases. When characteritics of transmission wavelength including such a plurality of peaks are obtained, it becomes required to produce a wavelength interval between two transmission peaks and transmittance strictly with high reproducibility. In an example in which an interference filter by a multilayer dielectric film is used, it is very difficult to produce such double-humped transmission characteristics strictly and stably.
Further, in an example in which a grating (a fiber type grating or a grating provided on a glass substrate) is used, the wavelength dependency of the loss has a steep peak at a specific wavelength and has a large lag from the gain wavelength curve of the opticalamplifier. Therefore, when it is used for compensation of the gain wavelength dependency, there is such a drawback that a compensation error becomes larger in point of principle.
There is a Mach-Zehnder type optical filter as another conventional example. In this filter, a Mach-Zehnder type waveguide is formed on a quartz waveguide substrate, and a thermal optical effect is used for respective phase adjustments. In this Mach-Zehnder type optical filter, a refractive index change by the thermal optical effect is generated by adjusting the voltage applied to an electrode, thereby to change the phase so as to adjust the transmittance wavelength dependency in a wavelength direction. The Mach-Zehnder type optical filter is described in detail in Japanese Patent Application No. Hei 5-60047 for instance.
In the case of a Mach-Zehnder type optical filter, however, phase adjustment of a transmission light is made by generation of heat in an electrode provided on a waveguide. Therefore, there is such a drawback that a distortion inside the waveguide is liable to be generated, which generates polarization dependency of insertion loss.